1. Field of the Invention
The present invention relates to an epoxy functional silicon resin used in an addition curable silicone release coating composition. More particularly, the invention relates to an epoxy functional silicone resin controlled release additive containing tri- or quadri-functional siloxane units, for use in a vinyl-functional base silicone release coating, thereby providing a thermal and addition curable, controlled release coating composition.
2. Technology Review
Silicone compositions have become widely accepted as release coatings, which are useful to provide a surface or material that is relatively non-adherent to other materials which would normally adhere closely thereto. Silicone release compositions are useful as coatings with release pressure sensitive adhesives for labels, decorative laminates, transfer tapes, etc. Silicone release coatings on paper, polyethylene, and other such substrates are also useful to provide nonstick surfaces for food handling and industrial packaging.
Silicone release coating compositions are well known in the art. Traditionally, non-functional and vinyl-functional silicone resins have been used as controlled release additives for silicone release coatings. In general, a resin and/or fluid mixture of vinyl-functional silicone and SiH-functional silicone cross-linking agent is cured in the presence of a precious metal catalyst by the well known hydrosilation addition reaction. By varying the amount of vinyl-functional silicone resin added to base silicone composition, a variety of release levels can be obtained from a given system.
Presently release coatings are of two basic types, heat cured and UV cured. Heat cured release coating compositions contain terminally and/or on-chain vinyl-substituted silicone polymer, SiH functional silicone cross-linking agent, hydrosilation addition reaction promoting catalyst and addition reaction inhibitor. Heat is employed to deactivate the inhibitor and allow the hydrosilation addition reaction to proceed.
There is a continuing need in the release coating industry to develop materials with "differential release" characteristics. In other words, there are wanted release coatings, the release of which may be adjusted to various levels as desired. With "differential release", premature release can be prevented while accounting for easy release when desired.
U.S. Pat. No. 4,448,815 discloses a heat curable release coating composition crosslinked by addition reaction in the presence of platinum catalyst. There is no teaching as to a controlled release.
U.S. Pat. No. 4,576,999 discloses a UV radiation curable release coating composition crosslinked by addition reaction in the presence of platinum catalyst. Again, there is no teaching as to controlled release.
U.S. Pat. No. 4,547,431 discloses UV radiation curable epoxy-functional silicone further containing poly-functional epoxy monomers. The additional monomers provide quantitative variable release. In this case, heat cure to produce a release coating was not reported.
U.S. Pat. No. 4,684,670 discloses UV curable silicone terpolymers wherefrom differential release was achieved, but control of this differential release was not obtained.
U.S. Pat. No. 4,952,657 discloses UV curable, phenol-modified epoxysilicones as controlled release compositions. It is disclosed that the greater the content of phenol-modified silicone resin, the greater the release value that is obtained.
Commonly-assigned U.S. application Ser. No. 07/573,416, filed Aug. 24, 1990, now U.S. Pat. No. 5,158,991 discloses UV-curable epoxysilicones for producing controlled release coatings. The utility of the incorporation of tri- and quadri-functional siloxane units with respect to the coating is not disclosed.
There may also be employed in release coating compositions additional silicones for their ordinary purpose which contain no siloxane units with a release modifying substitution. For example, an unmodified hydride-substituted silicone may be employed to perform the major part of the crosslinking function. Thus, it is not critical that each polymer of a release coating composition be modified. It is critical, however, that there be sufficient release modified organopolysiloxane with sufficient units having release modifying substitution to increase release in the cured composition.
There is thus a need for additional silicone release coating additives wherefrom stable controlled release can be obtained. In particular, there is a need for controlled release coatings which are economical, easy to manufacture from readily available materials, and which provide for greater levels of release than traditional vinyl-substituted additives, and be solventless to minimize dealing with solvents and solvent recovery. This resulting in a more environmentally sound product.